The present invention relates to developing new high-efficiency evaporators.
In refrigeration equipment, air conditioning equipment and heat pumps, commonly named heat pumping equipment, it is very important to operate with small temperature differences between the heat source, e.g. air or water, and the boiling refrigerant in the evaporator. These small temperature differences contribute to decrease the difference between the condensation temperature and the evaporation temperature which is very important to achieve high energy efficiency of the system, usually expressed in terms of the coefficient of performance (COP) defined as, for heating purposes, the amount of heat (q1) delivered to the warm side divided by the amount of work (ε) required for the compression of the refrigerant vapor (COP1=q1/ε), and for refrigeration purposes as the amount of heat (q2) absorbed from the cold side divided by the amount of work (ε) required for the compression of the refrigerant vapor (COP2=q2/ε).
The heat transfer rate in evaporators is governed by the equation Q=h·A·ΔT, where h is the heat transfer coefficient (HTC), A is an area relating to the heat transfer surface and ΔT is the temperature difference between the surface and the bulk fluid. To achieve low temperature differences, a high HTC or a large heat transfer surface area is needed. Thus, to reduce the temperature difference in the evaporator of heat pumping equipment some type of enhanced surface can be used which can promote bubble nucleation and thereby increase the HTC of the evaporator.
The enhancement could also be a mean to reduce the necessary size of the evaporator, without increased temperature difference, for miniaturization purposes (smaller, more space efficient and economical evaporators). Enhanced surfaces not only increase the heat transfer coefficient but may also increase the critical heat flux (CHF) and decrease the temperature overshoot at boiling incipience. CHF is a decisive parameter when designing cooling solutions for applications with high heat flux, such as cooling of electronic components and safety systems in nuclear power reactors.
A decreased temperature overshoot at boiling incipience results in a significantly higher HTC at low heat flux and is therefore desirable in many applications (electronics cooling at low heat flux, heat pumping technology, etc.).
Such enhanced surfaces for nucleate boiling have received considerable attention during the last decades and are frequently identified as “high performance nucleate boiling surfaces”
During the past few decades, several investigations have been completed concerning the issues associated with high performance nucleate boiling surfaces. These surfaces could be manufactured either by mechanical methods or by chemical methods. Mechanical methods include the surface deformation techniques such as abrasive treatment and inscribing open grooves. Chemical methods would further be subdivided into two types; the first type being surface erosion techniques like electrolysis and chemical etching while the second type refers to the coating of a porous layer of chosen material on the boiling surface. This coated layer can be fabricated in many ways, such as sintering, spraying, painting, electroplating, etc.
However, little attention has been paid to the surface modification by nanostructuring to produce high performance nucleate boiling surfaces.
The prior art of surface modification for enhanced heat transfer in boiling used methods based on mechanical deformations or physical methods such as spraying particles to surfaces. Those methods are not capable of creating well defined nanostructured surfaces, because of the physical limitations of the mechanical techniques, and are therefore limited to the creation of less well-defined micron-sized features.
Since much of known technology has been limited to the micron-scale region, the focus of boiling research has primarily been to investigate the micron-scale influence on the boiling characteristics of a surface or an enhancement structure. Nanoscale features like surface roughness, grain boundaries, cavities between nanoparticles, rather than micron-scopic cavities on the heater surface, may have been responsible for the reduced nucleation energy barrier observed at the onset of nucleate boiling. Hence, to create an efficient boiling surface it is important to be able to control both the micron- and nano-scale features of the evaporator surface.
U.S. Pat. No. 4,216,826 disclose an enhanced boiling surface on a tube, which has been mechanically fabricated by deforming, compressing and knurling short integral tube fins. Since the structure can only be fabricated on circular geometries, the area of application is limited to boiling on the outside surface of tubes. The mechanical treatment also prohibits the possibilities for tailor making the nano-features of the structure.
U.S. Pat. No. 3,384,154, U.S. Pat. No. 3,352,3577 and U.S. Pat. No. 3,587,730 disclose enhanced boiling surfaces, well know commercially as the “High-Flux” surface, fabricated by sintering of metallic particles to surfaces and thus creating a porous coating. This fabrication technique is restrained to producing randomly sized cavities and with limited possibility to modify the nano-sized features of the structure. Thus, the structure is not well ordered and it is not possible to tailor make features in the nano-scale to enhance heat transfer in boiling.
JP 2002228389 relates to a heat transfer promotion approach wherein performing surface treatment which forms the boiling heat transfer side with concave convex protruding parts of the height of 10 nm to 1000 nm. The surface may consist of different metals such as aluminum and is fabricated using CVD technique or sputtering techniques followed by wet etching.
U.S. Pat. No. 4,780,373 relates to a heat transfer material for boiling produced by electrodeposition method, where a dense porous layer is formed which has dendritic miniscule projections densely formed on the surface. The layer has an average thickness of 50 μm.
Approximately 15% of all electricity produced is used for running heat pumping equipment. For each degree, the temperature difference between the heat source and the evaporating fluid is reduced; the electricity need for running the system is reduced by 2-3%. Accordingly, there is a need of enhanced surfaces in the field of heat transfer in boiling. It is an objective of the present invention to provide a surface which could be used for enhancing heat transfer in boiling as well as a new method for forming a new surface.